This compound is called an olefin or alkene. The modern designation of trans (cis) refers to the arrangement of substituents on a ring. Historically, the term was applied to olefins as well. When the substitution pattern is complex, the system breaks down. (E)- and (Z)- are absolute designations. (E)-and (Z)-butene are referred to as geometrical isomers. In fact, they are diastereomers. Any stereoisomer that is not an enantiomer is a diastereomer independent of whether or not stereogenic atoms are present!